X-ray diffraction has been used to quantify the interface roughness of GaAs/AlxGa1-xAs multilayers. X-ray measurements, both θ-2θ and rocking-curve analyses, quantitatively determine the correlated and random components of the interface roughness as well as the correlation length of the roughness along the interface. The multilayer structures of GaAs/Al0.3Ga0.7As and GaAs/AlAs differ substantially in the amplitude and correlation length of the interface roughness. The change in interfacial roughness is compared to that determined from low temperature photoluminescence (PL) measurements, the conventional measure of interface uniformity.

The evolution of interfaces of short-period (SimGen)p superlattices upon annealing has been studied by x-ray reflectometry and Raman scattering spectroscopy. Isothermal annealing treatments at 700° C resulted in a significant material redistribution as evidenced by a strong decay of the superlattice x-ray satellites and by the decay of longitudinal acoustic modes and changes in the optical mode intensity ratios in Raman scattering. Interdiffusion was more pronounced in superlattices of short periodicity. This may possibly be explained by the strong composition dependence of the diffusivity of Ge atoms in Sil-xGex alloys and the degree of preexisting interfacial mixing. This composition dependence of the diffusion may favor atomic displacement parallel to the interfaces leading to an initial smoothing of the interfaces

A mathematical model has been developed of the reactor used by Larsen et al. [1] to study the kinetics of gallium arsenide MOCVD.- Two different surface reactions were considered as the rate-limiting step in film growth below 500°C: (1) the desorption of methyl radicals from adsorbed trimethylgallium, and (2) the reaction of CH3 and H groups from adsorbed trimethylgallium and arsine to make methane. The latter step is consistent with the experimental results. It explains the rapid acceleration of the precursor decomposition rates when they are fed together to the reactor. It also explains why methane is the only hydrocarbon generated from trimethylgallium and arsine decomposition in deuterium at V/Ill ratios greater than 1.0.

(2×4) and c(4×4) reconstructed GaAs(001) surfaces prepared by Molecular Beam Epitaxy (MBE) were studied by Reflection High Energy Electron Diffraction (RHEED). A method for accurate determination of the Arsenic coverage of reconstructed GaAs(001) surfaces is introduced. The time of Gallium supply to the reconstructed surface until a halo appears in the RHEED pattern is taken as measure for the Arsenic coverage. Structures between them were investigated at a substrate temperature of 200°C. The RHEED results were verified by High Resolution Scanning Electron Microscopy (HRSEM). Dependent on the surface reconstruction, Arsenic coverages between 0.76 and 1.22 monolayer (ML) were observed. Surface structures, observed during transformation between β(2×4) and c(4×4) will be discussed.

Chemical Vapor Deposition often occurs under conditions unsuitable for simultaneous monitoring using electron-based structural probes. This is due to the near-atmospheric pressure which exists during growth. While some work has been performed using optical probes, the typically high temperatures in the reactor vessel and the vastly longer wavelength of visible light versus the features being studied has hampered its utility as a structural probe. Over the past several years x-rays from synchrotron storage rings have been used to study the surface structure of growing materials. These studies have been primarily scattering studies. In this paper we explore the use of x-ray spectroscopic studies for the analysis of film growth. We have focused on three areas: the measurement of reactor vessel profiles using x-ray fluorescence; The measurement of gas-phase Extended X-ray Absorption Fine Structure (EXAFS), and the measurement of EXAFS of adsorbed species on the substrates.

In this paper, we study the significance of a low temperature nucleation layer and its role in inhibiting interfacial pitting vapor deposition. Transmission electron microscopy and scanning electron microscopy studies show that severe interfacial pitting occurs for thin nucleation layers of average thicknesses of 200Å and 400Å. For these average nucleation layer thicknesses we have found that the InAs islands do not cover the entire substrate surface during the low temperature deposition. Hence, when the film is heated to a higher temperature for the growth of the remainder of the film severe pitting at the heterointerface is produced. The thermal etchpits are sources of threading dislocations, which propagate to the surface of the film. For thicker nucleation layers we observe no interfacial pitting. Our studies show that there is an optimum nucleation layer thickness for which high quality InAs films with reduced threading dislocation densities and relatively high electron mobilities are obtained. Both electrical and structural studies suggest that ∼ 800Å is an optimum thickness of the low temperature nucleation layer.

Two new organometallic Sb sources, tris(dimethylamino)antimony (TDMASb), and tertiarybutyldimethylantimony (TBDMSb) are being investigated. The growth of InSb using TDMASb, or TBDMSb and TMIn was investigated over a temperature range of 350 to 475 °C using a range of V/III ratios, growth rates and pressures. Misoriented InSb substrates were also used for selected growth conditions The growth rates of lnSb using TMIn and either TDMASb or TBDMSb at temperatures <= 425 °C were proportional to both the TMIn flow rate and the temperature. The surface morphology of InSb grown using either TDMASb or TBDMSb was very rough for growth temperatures <=425 °C. This may be due to the complex decomposition mechanisms involved and the presence of methyl groups on the surface. Smoother backgrounds were generally found when using off axis substrates. The details of the defects observed on the surfaces were dependent on the type of misorientation and can be related to the atomic structure of the surface steps. Both n- and p-type InSb were grown using TBDMSb or TDMASb and TMIn with mobilities up to 68,990 and 7773 cm2/Vs, respectively, at 77 K. The mobility for lnSb using either TDMASb or TBDMSb was optimized by going to lower temperatures, pressures and V/III ratios. The opposite was true for surface morphology which improved with higher temperature, pressure, and V/III ratio. The growth of high mobility InSb with smooth surfaces at T< =425 °C was not achieved with TDMASb or TBDMSb and TMIn under the conditions investigated in this work.

The crystal quality of III-V substrates was studied by x-ray asymmetric crystal topography (ACT). An extensive study of device-quality GaAs (LEC SI, LEC n-type, VGF and VB) and InP (LEC n-type (S) and SI (Fe) ) wafers reveals that they can contain various defects: low angle grain boundaries, dislocation cell structure, slip, swirl and included mosaic structure.

The high angular resolution of ACT was used to non-destructively study both the epitaxial layer and the substrate. Topographs of the substrate and the epitaxial layers of AlGaAs/GaAs layers grown by MBE and MOCVD are compared. The quality of the substrate has direct implications for the quality of the epitaxial growth. For coherent layers, both the large scale and small scale defects observed in the substrates were exactly reproduced in the epitaxial layers grown by both techniques.

CoSi2 was formed by annealing a multilayer strucure of sputter deposited Co / Ti / Si. The Ti was 1, 2 or 5 nm thick and the Co was 15 nm thick. The morphology of both the surface and the CoSi2 / Si interface was examined. For the sample that had 2 nm Ti layer, an epitaxial CoSi2 was formed with both a planar surface and a planar CoSi2 / Si interface. An amorphous Ti-Si-O layer was initially formed between the Co and the Si. Co then diffused through this layer and directly formed epitaxial CoSi2 on the Si<100>. No intermediate silicide phases of Co were observed.

The growth mechanism of YBa2Cu3O 7-= thin films grown by the BaF2 post annealing process at low oxygen partial pressure have been studied by transmission electron microscopy. Under the annealing conditions of po2 = 4 Torr and 700°C, BaCuO2 and Y2 Cu2O5 precipitates develop from stoichiometric film precursors of YBCO during annealing. A growth model is proposed based on the observations. In addition, early stage nucleation and growth of both c- and a-axis oriented grains at the substrate interface were observed in quench annealed cross-sectional samples. 90° [100] symmetrical boundaries form between the a- and c-axis oriented grains. The possible effects of these boundaries are discussed.

Thin (0.5 to 8 triple layer) CaF2 on Si(111) films were grown using molecular beam epitaxy (MBE) and characterized using an in situ combination of x-ray photoelectron spectroscopy (XPS) and componentresolved x-ray photoelectron diffraction (XPD). We identified surface, bulk-like, and interface F and Ca core-level shifts and used the XPS shifts and XPD modulations to identify the growth modes as a function of the kinetic parameters of CaF2 flux and Si temperature. We identify 3 distinct regimes: (i) for high temperatures and flux we find a complete reacted F-Ca-Si layer, overlaid by 2 layer high islands which coalesce, followed by layer-by-layer growth, (ii) for high temperature and low flux, we find the reacted F-Ca-Si layer to be partially covered with thick islands, and (iii) for low temperatures we find an incompletely occupied F-Ca-Si layer followed by layer-by-layer growth. In all cases we find the buried interface to be structurally identical to the unburied F-Ca-Si layer

We present evidence for two types of ordered structures, CuAu-I and CuPt, in Zn1−xFexSe (x≈ 0.4) epilayers grown by molecular beam epitaxy. These ordered structures are observed in both electron diffraction patterns and cross-sectional high-resolution lattice images. The CuAu-I ordered structure occurs in Zn1−xFexSe epilayers grown on (001) InP substrates, while the CuPt-type occurs in epilayers grown on (001) GaAs substrates. The ordered structure of Zn1−xFexSe grown on InP substrates consists of alternating ZnSe and FeSe layers along the [001] growth direction and the [110] direction. In contrast, the ordered structure of Zn1−xFexSe grown on GaAs substrates consists of alternating ZnSe and FeSe layers along the < 111 > directions. We have also investigated the role of the misfit strain associated with the lattice mismatch between the epilayers and the substrates on the type of ordered structure.

Scanning tunnelling microscopy (STM) has been used to image the adsorption of trimethylgallium (TMGa) on GaAs(001)-(2×4) surfaces prepared in situ by molecular beam epitaxy (MBE). Filled states images of the clean surface are dominated by (2×4) unit cells containing only two As dimers. Upon exposure of this surface to TMGa at room temperature, bright oval-shaped features are observed which are centred on the arsenic dimers of the unit cell. These arise from tunnelling from Ga-C bonds of the adsorbed molecules. At low coverages, preferential adsorption on unit cells adjacent to occupied sites along the [110] direction is observed. A detailed statistical analysis of a large number of adsorption sites shows that there is an increased probability of about 24% for adsorption next to a (2×4) unit cell which is occupied relative to an unoccupied one.

The reactions of O2, H2O, CO2, NO2, NO, and N2O with single crystal graphite between 400 and 700°C have been studied by STM to obtain quantitative kinetics by measuring the number and size of monolayer pits on the basal plane versus temperature and time. At low temperature the reaction initiates exclusively from the point defects on the basal plane to form monolayer pits. The shape of the monolayer pits vary from nearly triangular to hexagonal to circular depending on the rate of the reaction and the reacting gases. The sizes of the monolayer pits grow linearly with reacting time. The monolayer reaction rates follow the order of RNO2 > RN2O > RNO > RO2 > RH2O > RCO2. The activation energies for reactions with O2, H2O, NO2, NO, and N2O, are determined to be 127, 205, 60, 89, and 74 kJ/mol respectively.

Carbon deposition from hydrocarbons onto surfaces of single crystal graphite has been examined to study the fundamental steps of chemical vapor deposition. Uniform monolayer pits on graphite surface were first produced by reactive etching of freshly cleaved single crystal of graphite in oxygen and carbon was then made to deposit exclusively on these defects in the basal plane. Carbon vapor deposition forms unique structures around the monolayer steps. By measuring the sizes of structures on steps in various gases versus temperature and pressure, the kinetics of hydrocarbon decomposition and the role of surface diffusion can be determined.

Phosphorus exposed GaAs (100) surfaces during a Chemical Beam Epitaxy growth process are studied using in-situ Reflection High Energy Electron Diffraction and ex-situ High Resolution X-ray Diffraction. It is shown that the phosphorus exposure of a GaAs (100) surface in the 500 – 580 °C temperature range results in the formation of one GaP monolayer.

Low temperature growth of silicon from silane is required in devices such as germanium- silicon HBTs and multiple quantum well structures for infrared detectors. Understanding the silicon growth kinetics is an important step in developing a complete model for incorporation of other species such as germanium, boron, and phosphorus. In this paper, we show that a wide range of recent data can be well explained by a model which includes the effect of hydrogen carrier gas and is based on a simple two- step reaction sequence for silane decomposition. Gas phase reactions and gas phase transport limits appear unimportant at least up to 1 Torr total pressure.

The epitaxial growth of Ge on Si using Te as a surfactant has been studied with high resolution photoemission, low energy electron diffraction and cross-sectional transmission electron microscopy. The growth mode of Ge on Si changed from Stranski-Krastanov (S-K) to layer-by-layer mode when 1/4 ML Te atoms were on the surface. During the growth, Te atoms segregated to the top of the surface. If the growth temperature is too high (above ∼450°C), the Te coverage was less than that necessary to keep the layer-by-layer growth, and the growth mode of Ge on Si is still S-K.

Low temperature (T<150 K) RHEED oscillations during the growth of ultrathin films suggest the presence of substantial adatom mobility. In most systems thermal diffusion can not account for the observed oscillations, and the origin of the oscillations is an open question. Experiments on Ag/Si(111) at 150 K demonstrate the absence of thermal diffusion due to the observed scaling in the RHEED intensity for different flux rates. We have performed molecular dynamics simulations to understand the mechanisms of RHEED oscillations at low temperature. Classical two- and three-body Si potentials were used together with an adatom mass that is 3.84 times the Si mass to account for the Ag/Si mass ratio. Results indicate that the landing site for the adatom is very important to predict whether a particular adatom will display lateral motion or not. A fraction of the adatoms incident near a maximum of the potential energy surface display significant lateral motion. The substrate stiffness for Ag/Si results in an energy transfer process which is much slower than that in fcc/fcc systems.

We present results of Monte Carlo simulations of irreversible diffusion-mediated nucleation and growth of square islands during deposition. This model mimics metal-on-fcc(100) metal epitaxy at lower temperatures. Our analysis focuses on the scaling of the island size and separation distributions, and their evolution with coverage. The depletion in the density of nearby island pairs is shown to produce a “Henzler ring” structure in the diffraction intensity profile.

The recent discovery of reentrant layer-by-layer growth in metal homoepitaxy has stimulated considerable interest in the role played by barriers to hopping down descending steps. However, the existence of step-edge barriers for semiconductors is far from being clearly established. We have investigated the effects of step-edge barriers for epitaxial growth and the “inverse” process of low-energy ion sputtering on metal surfaces using Monte Carlo simulations of a solid-on-solid model. Our results are in good agreement with available experimental data and provide new insights into the microscopic origins of the evolution of surface morphology during these processes. Our simulations also suggest that such step-edge barriers can explain the observed temperature and time dependence of the reflection high-energy electron diffraction intensity during post-growth recovery on GaAs(001), which sheds new light on this controversial subject.